I have a laptop with an external monitor connected via HDMI (output name is
HDMI1). The command
xbacklight works only with my primary screen (which is the laptop screen).
It doesn't work even if I try running it with argument
-display HDMI1 (which gives me an error
RANDR Query Version returned error -1) or
--display HDMI1 (which gives me the help information).
I have seen this - Turn off the backlight of an external monitor - question but it only mentions how to change the brightness, not backlight.
EDIT: the option
-display is not documented in the manual page of
xbacklight, but after a quick look into the source code I've found where it's used. In the function
xcb_connect it represents the X display, not the actual screen name.